#include <iostream>
#include <string>
#include <cstdlib>
using namespace std;

int maxcmp(const void *a,const void *b)
{
    printf("OK");
    return *(char *)a-*(char *)b;
}

int main()
{
    string a;
    int n,i,len,sum;
    cin>>n;
    while (n--)
    {
        sum=0;
        cin>>a;
        len=a.size();
        qsort(&a,len,sizeof(a[0]),maxcmp);
        cout<<a;
    }
    return 0;
}

#include <iostream>
#include <string>
#include <cstring>
#include <cstdlib>
using namespace std;

int cmp(const void* cp1, const void* cp2) {
   return *(char*)cp1 - *(char*)cp2;
}

int main() {
   string str;
   char* temp;
   int n;

   cin >> n;
   while (n--) {
      cin >> str;
      temp = new char[str.size() + 1];
      strcpy(temp, str.c_str());
      qsort(temp, str.size(), sizeof (char), cmp);
      str = temp;
      cout << str << endl;
      delete temp;
   }
}

似乎只能用这个方法了，何不直接用char[]？这样似乎还更有效。

qsort(&a,len,sizeof(a[0]),maxcmp);
改为
qsort(&a[0],len,sizeof(a[0]),maxcmp);
试试看
我没试